This invention relates to a motor rotation speed controlling apparatus, and more particularly, to an improved motor rotation speed controlling apparatus which controls the rotation speed of a motor whose load changes frequently and which is employed in electric tools such as an electric screw driver, electric drill, or electric saw.
In a conventional motor rotation speed controlling apparatus, a feedback circuit is used that employs a tachometer to control the speed and torque of a motor, for example, a d.c. motor, during a load running of electric tools such as an electric drill. Such a conventional speed controlling apparatus, however, requires a motor current detecting circuit, a differential amplifier, and a tachometer to detect currents flowing through the motor and to measure the rotation speed thereof. Therefore, the conventional apparatus has the following disadvantages: it requires an increased number of electrical parts, it has a high cost, it involves a complicated circuit configuration, and it is difficult to change the characteristics of the motor.